Everyone with belt questions/issues should look at
this thread and
this thread. Also, I
wrote a post in the first thread (long I know) about what 971 has learned about belts.
I summary: use Gates GT2 and not the Vex HTD. It's my experience that they are better in terms of manufacturing quality and usefulness for FRC applications. Also, look up Gates/other companies documentation. There is quite a bit of information out there if you know where to look (I linked some in my original post). Knowing what a properly tensioned belt is is important, and knowing the signs of an undertensioned or overtensioned belt can save you many headaches in the future.
Andrew,
One question I have for your belt failures is how high is your center of gravity? 971 speculates that having a high cg could affect the failure of drivetrain belts. (see my post for more details)
If you post pictures of your belt failure, I or someone else might be able to help you diagnose what happened.